1. Field of the Invention
The present invention relates to optical filters and beam generators and, in particular, to spatial optical filters for obtaining substantially plane-wave reference beams that have temporal coherence with a prespecified information bearing beam of optical light.
2. Description of Related Art
One application for optical filters and beam generators is in optical holography which is a technique for obtaining phase information from a signal light beam. The basic principles of holography are well known for infrared, visible and ultraviolet light sources. A hologram is formed when two light beams are caused to interfere within a holographic medium, that is, a medium which can be made to record the intensity distribution o an incident light field. Typically, a hologram is formed when a signal beam carrying phase information interferes with a reference beam. The reference beam comprises a plane wave or nearly plane wave light, that is, light having a substantially constant phase and intensity across its wavefront. For the signal beam to interfere with the reference beam, the two light beams must be temporally coherent with each other.
For the purposes of this application, two light beams are temporally coherent if, when the beams are combined, the beams will interfere to form interference fringes. It is recognized that this definition encompasses a range of coherence from beams that are perfectly coherent to beams that are only partially coherent, but have sufficient spatial and temporal coherence to form interference fringes.
One well known way of obtaining a reference beam that is spatially and temporally coherent with a signal beam is to derive the reference beam from the same light source as that used to form the signal beam. This technique is described herein in a simplistic fashion to illustrate the very general area of the problem to which this invention is directed. In certain types of holography, lasers are used as light sources because they provide monochromatic light with a long temporal coherence length. Light from a laser beam is spatially filtered and then passed through a beam splitter to form two beams, one of which becomes the signal beam, the other of which becomes the reference beam. At this point both beams are plane waves. The signal beam then travels along some optical path in which it either passes through an aberrator or is reflected off an aberrator. In this process the aberrator modulates the signal beam so that phase or intensity information characteristic of that aberrator is incorporated into the signal beam. After the signal beam interacts with the aberrator, it is usually no longer a plane wave. The signal beam is then directed to the holographic recording medium where it will be combined with the reference beam.
After the reference beam leaves the beam splitter, it travels an optical path whose length is adjusted to match the length of the signal beam path. The lengths of the respective paths travelled by the signal beam and the reference beam are matched to ensure that the beams remain temporally coherent. Ideally, the reference beam should not interact with the aberrator or any other medium which might introduce a phase variation across the wavefront of the reference beam. The reference beam thus remains a plane wave until it is directed to the holographic recording medium where it combines with the signal beam, creating interference fringes within the holographic medium, and forming a hologram. The hologram then bears information characteristic of the aberrator and the hologram can then be used to recreate the image of the aberrator.
While such an independent reference beam can be used in a laboratory environment, such a technique may not be feasible when the holography is performed over large distances. For example, it may be impractical to match the reference beam's optical path length to that of the signal beam. Additionally, such a technique may yield a reference beam that is not optimally suited to this type of holography. Optical delay lines can be used to create a local reference beam, but a reference beam produced in that fashion may not be ideal for extracting information about a particular aberrator from the signal beam. This is because frequency or phase shifts might be present in the signal beam that will not be present in a local reference beam. When the signal beam interferes with the local reference beam, any frequency shifts or phase shifts that have occurred along the signal beam optical path will degrade the quality of the resultant interference fringes.
An example of such a phase or frequency shifting problem occurs in the atmosphere. The atmosphere can act as an aberrator, diminishing the spatial coherence of a signal beam with a local reference beam. A light beam back scattered from the atmosphere can be Doppler shifted and or Doppler broadened sufficiently to affect the coherence of that beam with a reference beam generated in an optical delay line. If the signal beam and the local reference beam travel substantially different paths, the two beams may not produce strong interference fringes when they are combined.
One technique for addressing this problem is to derive the necessary reference beam from the signal beam after the signal beam has travelled whatever optical path it is constrained to travel, and has interacted with the various aberrators along the signal beam optical path. This technique is known as "self-referencing" because holograms can be produced without having an independent reference beam travel a path similar to that travelled by the signal beam. In the self-referencing technique, a portion of the signal beam is split off from the signal beam by, for example, a beam splitter. The split off beam is then spatially filtered to remove the phase information from that beam, creating a plane wave that is spatially and temporally coherent with the signal beam. This beam can then serve as a reference beam which, when combined with the signal beam, will interfere so that a hologram can be formed.
A substantial disadvantage with this technique is that self-referencing cannot typically be performed with low light level signals. This is because losses of 99% or greater are associated with the spatial filtering typically performed to obtain the reference beam. Optimal conditions for holography require that the power in the signal beam and the reference beam be equal to maximize the contrast between fringes. Thus a 99% loss in reference beam intensity will cause a greater than one order of magnitude drop in the signal available to form the hologram.
In one prior art self-referencing technique, the beam split off from the signal beam is spatially filtered with a filter comprised of two lenses and a pinhole as shown in FIG. 1. In operation, an incident beam of light is focussed at the pinhole by one lens. The diameter of the pinhole is typically chosen as the diffraction limited spot size associated with the wavelength of the light in the signal beam and the size and focal length of the second lens of the spatial filter. The pinhole is usually located at the focus of the first lens, i.e., at a distance equal to the focal length f.sub.1 of that lens. After the beam of light passes through the first lens and the pinhole, the beam is collimated by passing it through the second lens. The second lens is positioned so that the pinhole is at the focus of the second lens, that is, the distance between the pinhole and the second lens is equal to the focal length f.sub.2 of the second lens.
Typically, the spatial filter illustrated in FIG. 1 must be carefully aligned because the light intensity throughput usually drops rapidly for even small misalignments. Also because of the criticality of the alignment of this device, this spatial filter can be prone to losses due to misalignments which occur as a result of vibrations. Yet another disadvantage of this spatial filtering technique is the vulnerability of the pinhole to damage. In this spatial filter, the beam of light is normally focussed to as small of a spot as is possible, creating a high level of optical power density at the pinhole. For high power applications, this can cause damage to the pinhole.
This spatial filtering technique has a further, often critical disadvantage when used to produce a self-referencing beam for holography of distant or atmospheric objects. Light which passes through a strongly-aberrating medium is subject to rapid variations in spatial phase caused by variations in the density of the air through which the light passes. For light that has passed through the atmosphere, these variations in phase will cause rapid variations in the observed optical intensity which are known as scintillations. When a self-referencing beam is derived from a beam of light that has passed through the atmosphere, the beam will tend to drift in position and will suffer from strong intensity modulation at the spatial filter plane. These scintillations result in an often unacceptable level of variation in the reference beam power, limiting the self-referencing beam's usefulness for holography.